Pools: Make `PermissionlessWithdraw` the default claim permission (#3438)

Related Issue https://github.com/paritytech/polkadot-sdk/issues/3398

This PR makes permissionless withdrawing the default option, giving any
network participant access to claim pool rewards on member's behalf. Of
course, members can still opt out of this by setting a `Permissioned`
claim permission.

Permissionless claiming has been a part of the nomination pool pallet
for around 9 months now, with very limited uptake (~4% of total pool
members). 1.6% of pool members are using `PermissionlessAll`, strongly
suggesting it is not wanted - it is too ambiguous and doesn't provide
guidance to claimers.

Stakers expect rewards to be claimed on their behalf by default - I have
expanded upon this in detail within the [accompanying issue's
discussion](https://github.com/paritytech/polkadot-sdk/issues/3398).
Other protocols have this behaviour, whereby staking rewards are
received without the staker having to take any action. From this
perspective, permissionless claiming is not intuitive for pool members.
As evidence of this, over 150,000 DOT is currently unclaimed on
Polkadot, and is growing at a non-linear rate.